DESCRIPTION

This function configures at runtime which SSL backend to use with
libcurl. This function can only be used to select an SSL backend once,
and it must be called beforecurl_global_init(3).
The backend can be identified by the id (e.g. CURLSSLBACKEND_OPENSSL).
The backend can also be specified via the name parameter for a case
insensitive match (passing -1 as id). If both id and name are
specified, the name will be ignored.
If neither id nor name are specified, the function will fail with
CURLSSLSET_UNKNOWN_BACKEND and set the avail pointer to the NULL-
terminated list of available backends. The available backends are those
that this particular build of libcurl supports.
Since libcurl 7.60.0, the avail pointer will always be set to the list
of alternatives if non-NULL.
Upon success, the function returns CURLSSLSET_OK.
If the specified SSL backend is not available, the function returns
CURLSSLSET_UNKNOWN_BACKEND and sets the avail pointer to a NULL-
terminated list of available SSL backends. In this case, you may call
the function again to try to select a different backend.
The SSL backend can be set only once. If it has already been set, a
subsequent attempt to change it will result in a CURLSSLSET_TOO_LATE.
Thisfunctionisnotthreadsafe. You must not call it when any other
thread in the program (i.e. a thread sharing the same memory) is
running. This doesn't just mean no other thread that is using libcurl.

AVAILABILITY

This function was added in libcurl 7.56.0. Before this version, there
was no support for choosing SSL backends at runtime.

RETURN VALUE

If this function returns CURLSSLSET_OK, the backend was successfully
selected.
If the chosen backend is unknown (or support for the chosen backend has
not been compiled into libcurl), the function returns
CURLSSLSET_UNKNOWN_BACKEND.
If the backend had been configured previously, or if
curl_global_init(3) has already been called, the function returns
CURLSSLSET_TOO_LATE.
If this libcurl was built completely without SSL support, with no
backends at all, this function returns CURLSSLSET_NO_BACKENDS.